1. Field of the Invention
The present invention relates to environmentally stable electrically conducting organic polymers and, more particularly, complexes of processible poly(3,6-N-alkylcarbazolyl alkene) doped with charge transfer acceptors together with a method of making same.
2. Description of the Prior Art
High molecular weight organice polymer materials are generally non-conductive because they do not have free electrons like metals. It has been found, however, that certain high molecular weight materials having intrinsic double bond structures such as polyacetylene, polythiazine and polypyrrole may become highly conductive when doped with certain electron acceptors or donors. These compounds have proved to be of a great deal of interest inasmuch as they may combine some of the traditional properties of organic polymers such as high strength, lightweight, flexibility and low temperature processing together with selective electrical properties including high electrical conductivity. In addition, their cost is relatively low.
Such materials undoubtedly will have an important impact on many areas of technology, especially the electronics industry. For example, experimental batteries made from conducting polymers have been shown to exceed current power sources in both power and energy densities. Other areas of potential applications include chemical or gas sensors, low cost, large area optical sensors, switches, lighweight electrical connections, wire, and in their film form for many types of microelectronic circuits and large area solar cells.
Thus, organic materials that behave as metals or semiconductors will provide the advantages of these materials together with additional advantages of being soluble in organic solvents or having low melting points and glass transition temperatures which both minimize the cost of processing and permit composites to be made with thermally sensitive materials such as doped Si or GaAs, for example. The enormous molecular design flexibility of organic chemistry enables precise tailoring of properties to fill a wide range of applications as enumerated above. In addition, the high strength and conductivity-to-weight ratios lend the advantage of fabrication of many electrical devices of much lower weight than conventional materials.
In the prior art, a large number of polymeric conductors have been made. These include polyacetylene and its analogues which may be doped with I.sub.2, AsF.sub.5 and BF.sub.4.sup.- or the like. In addition, various phenylene polymers and phthalocyanine complexes have been synthesized as conductive materials.
Highly conducting p-type materials have been obtained by doping the polymer with a charge transfer acceptor such as I.sub.2 or AsF.sub.5 from the gas or with ClO.sub.4.sup.- or BF.sub.4.sup.- by electrochemical oxidation. An n-type material has been achieved by a doping with alkali metal. In known cases of these two types of materials, however, to date only the p-type show any environmental stability.
Theoretically, conductivity takes place both along the polymer chain and between adjacent chains. The active charge carrier, at least in the aromatic materials, is believed to be a bipolaron that is delocalized over several monomer units. The mobility of such a species along the polymer chain is reduced by conformational disorder, necessitating a rigid highly crystalline chain structure for maximum intrachain conductivity. Various mechanisms such as "hopping" and interchain exchange" are thought to be responsible for the interchain part of the conductivity. Unfortunately all of the most highly crystalline polymers of high conductivity are insoluble and infusable. Such is the case with the most common prior art conducting polymer, polyacetylene, which because of this, must be used in the same form as polymerized. In film form it becomes highly porous fibrillar networks which are tough, cheap, and can be electrochemically doped very rapidly. Polyacetylene films have been used in lightweight storage batteries and can also be used to make Shottky barriers which exhibit a photovoltaic effect.
Other slightly less conductive materials include doped poly p-phenylenes; however, poly p-phenylene can be processed only by powder metallurgical techniques, precluding thin film applications. Two solution processible polymers that are known to have been doped to high conductivities in the prior art are poly m-phenylene and poly m and p-phenylene sulfides. AsF.sub.5 which has a very high electron affinity has been used successfully to generate radical cations in these polymers. Unfortunately, these cations are so unstable that crosslinking and ring fusion reactions occur. This, together with high water sensitivity, greatly reduces the utility of the polymers.
Thus, in the prior art, because of the nonprocessibility of these base polymers, thin films and uniform doping have both been difficult to achieve. One attempt to remedy this difficulty consisted of co-evaporating biphenyl with AsF.sub.5 to simultaneously polymerize the biphenyl and subsequently dope the p-phenylene polymer on the substrate. This procedure has also been used with several aromatic and heteroaromatic monomers capable of undergoing Lewis acid induced oxidative polymerization with an active radical cation chain end. Invariably black insoluble films of somewhat undetermined composition have resulted. Conductivities as high as 10.sup.-2 /ohm-cm were reached, however. This process for generating thin films is somewhat similar to the solid state polymerization of evaporated S.sub.2 N.sub.2 thin films to a semiconducting (SN).sub.X of rather low environmental stability.
Other conducting polymers which have been electrochemically synthesized and simultaneously doped are polypyrrole type films which show conductivities as high as 10.sup.2 /ohm-cm, and are stable in air. Unfortunately, these films are also intractable and of somewhat indefinite composition.
Successful environmentally stable doped conducting polymers are described in U.S. Pat. No. 4,452,725 to S. T. Wellinghoff, S. A. Jenekhe (a co-inventor in the present application) and T. J. Kedrowski which is assigned to the same assignees as the present application. That application concerns conducting polymers of N-alkyl 3,6' carbazolyl chemically doped with charge transfer acceptor dopants such as the halogens.